Can someone please explain to me why this 20" rim setup wouldn't perform as well as the 19" rim setup detailed below? Let me explain that both the 19's & 20's are the same rim and same tire brand just different sizes.

Here is my conclusion. The 20" rim setup will perform better for 2 reasons. 1) The overall wheel & tire weight is 2lbs less. Not a big difference but a difference none the less.
2) Since the 20" tires weigh less than the 19's, even if the overall weight were the same, the 20's would perform better because there is less weight farther from the center of rotation. Since the overall tire diameter is roughly the same even though the rims are heavier the tire is the farthest point from the center. Having a lighter tire will perform better because of less rotational mass. Plus the overall package being lighter.

I'm no engineer but it logically makes sense to me that when you distribute the weight closer to the center of rotation it improves performance. Does this make sense? Thanks for any and all suggestions.

Can someone please explain to me why this 20" rim setup wouldn't perform as well as the 19" rim setup detailed below? Let me explain that both the 19's & 20's are the same rim and same tire brand just different sizes.

Here is my conclusion. The 20" rim setup will perform better for 2 reasons. 1) The overall wheel & tire weight is 2lbs less. Not a big difference but a difference none the less.
2) Since the 20" tires weigh less than the 19's, even if the overall weight were the same, the 20's would perform better because there is less weight farther from the center of rotation. Since the overall tire diameter is roughly the same even though the rims are heavier the tire is the farthest point from the center. Having a lighter tire will perform better because of less rotational mass. Plus the overall package being lighter.

I'm no engineer but it logically makes sense to me that when you distribute the weight closer to the center of rotation it improves performance. Does this make sense? Thanks for any and all suggestions.

It does make sense - less rotational inertia. In a disk, I = ½ MR^2. Since the radial distance is squared, the distribution of the mass is a greater factor in inertia than the mass itself.

nah, 19s will be slightly more predictable in terms of handling, giving a bit more progressive loss of traction comared to 20s, and more comfortable on top of that, although with these sizes comfort is a purely relative turn.

It's not all about being lighter. With no sidewall to lean on, you are asking for trouble once you start pushing - when car snaps, it will be instant. with more sidewall you'll have more warning, more time to correct, etc. I'd stay away from huge rims if i wanted to drive car fast. But if you just want big wheels for show - by all means, get 22s even. :P

18s are optimal performance tires for handling for the M3. All timed Nurburgring runs use 18s.

First: The effect of size is more important than mass when calculating the moment of intertia. Wheels (aluminum) are much more dense than tires (rubber and steel and...). In any wheel for the M3 the outer "cylindrical" barrel portion of the wheel is the part that is the dominant contributor to the moment of intertia (I = integral r^2 dm) since it is furthest from the axis of rotation. The tires horizontal cylindrical portions (tread portions) will be rougly equal comparing 18, 19 or 20. Is is the sidewall the becomes smaller or larger, again this part of the tire is closer to the rotation axis.

Second: Tire deformation. Tires need to deform just the right amount to grip, absorb bumps without loosing traction, brake, etc. 18s are known to do this just right. 20s have way too short of a sidewall for proper lateral or vertical defection. 19s are a decent compromise between looks and performance.

These effects are fairly small but will be measurable on a timed track with a good driver.

swamp, took words out of my mouth. I was just gonna add that 18s for M3 are best tyre to have performance wise. Well there you said it for me. the sidewall deformation balance is what you need, not a big bloc of metal spinning around.

18s are optimal performance tires for handling for the M3. All timed Nurburgring runs use 18s.

First: The effect of size is more important than mass when calculating the moment of intertia. Wheels (aluminum) are much more dense than tires (rubber and steel and...). In any wheel for the M3 the outer "cylindrical" barrel portion of the wheel is the part that is the dominant contributor to the moment of intertia (I = integral r^2 dm) since it is furthest from the axis of rotation. The tires horizontal cylindrical portions (tread portions) will be rougly equal comparing 18, 19 or 20. Is is the sidewall the becomes smaller or larger, again this part of the tire is closer to the rotation axis.

Second: Tire deformation. Tires need to deform just the right amount to grip, absorb bumps without loosing traction, brake, etc. 18s are known to do this just right. 20s have way too short of a sidewall for proper lateral or vertical defection. 19s are a decent compromise between looks and performance.

These effects are fairly small but will be measurable on a timed track with a good driver.

I think I'm going to have to respectfully disagree with your first point. A 1 inch larger diameter rim that is overall 1lb heavier with 3lbs less wrapped around the outside, rear wheel example, should rotate much easier. The rim will play a much lesser roll than what's wrapped around the outer edge of it because that extra 3lbs is at the farthest point. The 1lb is only an extra .5inch farther from center. The 3lbs of tire is much farther from center and is 3 times heavier than that 1lb of rim. Plus the overall weight of the rim & tire is 2lbs less than the 19.

If you look at the wheel & tire from the center out this is what you get. Take the 20" setup for example. The overall diamter is 26.73 or a radius of 13.365 inches. Zero to 9.5 inches from center they are the same(19 inch diameter). Then the next .5 inch is 1lb heavier. Then the next 3.365 inches is 3lbs lighter than the 19" setup.

I'm not so sure about your second point except for the fact that each car is engineered from the factory to each specific sidewall. Otherwise your argument wouldn't hold any water for the Porsche 911 GT3 or the new Corvette ZR-1. Both of which use tire diameter or sidewall depths which violate what you say. Thinner sidewalls allow the suspension engineers to more accurately dial in a suspension because there is less tire variable to compensate for.

The facts are that the large the rim, the smaller the side wall and with it the ride gets worse, the transition from grip to slip is less predictable and the steering feel is unpredictable with increased tramlining.

Try an M3 with 20" and then try one with stock 18". If you think the 20" still feels better then you are the type of people that 'show' is more important than anything else.

The facts are that the large the rim, the smaller the side wall and with it the ride gets worse, the transition from grip to slip is less predictable and the steering feel is unpredictable with increased tramlining.

Try an M3 with 20" and then try one with stock 18". If you think the 20" still feels better then you are the type of people that 'show' is more important than anything else.

Try and follow my logic and math posted above if you want an answer to your first question. I don't necessarily think that 20's will perform better except that I can't see why they shouldn't from what I stated above. I don't even necessarily disagree with the sidewall argument except that there are cars much faster than my E92 M3 that run 20's stock or use sidewalls that are lower profile.

I'm about to purchase a new set of rims and I'm curious why everyone seems to agree that 20's perform worse because they're heavier when in my example they clearly are not. No matter what I decide to do I'm keeping my stock 19's for track work. On the track I completely agree that function takes precedent over form. You don't have to look pretty as long as you get the job done. On the street, however, I would argue that form "could" take precedent over function since you shouldn't be taking the car or the tires to the limit of their abilities. Racing is for the track.

Once again, I'm not sold on 20's nor do I firmly believe that they will perform better OR worse than 19's. I'm simply trying to answer a question that has been bothering me.

My take.
Instead of Inertia, think moment forces (lever arm). Take a wheel and cut out a slice of the pie.
You hold a stick with a weight (the rim plane plus a bit of wheel sidewall) on it 9.5" long. In the other hand, you hold another stick with a weight slightly heaver (because the circumfrence difference of the rim is 6.28" longer) and your stick is 1/2" longer. Which one is takes more strength to hold up?
The only real difference is the additional weight of the sidewall on the shorter stick because the tread area of the tires will wash out.

It's this moment force that requires a bit more work to both accelerate and decelerate since you've shifted the heaviest part of the wheel further away from the hub.

Then there is the predictability of tires with short sidewalls as mentioned.

edit: Also keep in mind races are won in 1/10ths at a time. For road work / commutting... it doesn't really matter all that much because it's doubtfull you are looking for extra little edge. So it really comes down to roadmanners and using the additional sidewall as a bit of buffer from the imperfections of the roadway.

Try and follow my logic and math posted above if you want an answer to your first question. I don't necessarily think that 20's will perform better except that I can't see why they shouldn't from what I stated above. I don't even necessarily disagree with the sidewall argument except that there are cars much faster than my E92 M3 that run 20's stock or use sidewalls that are lower profile.

OK, don't think just because another car which happens to come standard with a larger diameter rim and thinner sidewall means the same will be true for the M3 because it won't. The M3's suspension was designed to use 18" as standard and the 19" are probably just inside to window of both comfort and performance but to move up in size with no doubt compromise on both.

Quote:

Originally Posted by AZE92M3

I'm about to purchase a new set of rims and I'm curious why everyone seems to agree that 20's perform worse because they're heavier when in my example they clearly are not. No matter what I decide to do I'm keeping my stock 19's for track work. On the track I completely agree that function takes precedent over form. You don't have to look pretty as long as you get the job done. On the street, however, I would argue that form "could" take precedent over function since you shouldn't be taking the car or the tires to the limit of their abilities. Racing is for the track.

Once again, I'm not sold on 20's nor do I firmly believe that they will perform better OR worse than 19's. I'm simply trying to answer a question that has been bothering me.

I am not wanting to put you off buying the wheels you have set your heart on, only wanting to know why bigger is better. It's things I have never got my head around. I'm a kind of function over form kind of a guy and prefer a perfect performance/balance/comfort combination above anything else. Maybe the roads I drive on are of a much worse quality to most of the US and probably the ones you drive on.

I think the point here is whether he is using the M3 a street car or track car. Second the speed limit is 60mph, I don't think the average car owner is to concerned about the marginal differnce between 19" & 20" wheels for a street car.

One is awhere that the ride becomes a little harsher with 20" wheels and that track drivers tend to own a track set of wheels anyway.

Everyone here owns a M3, either a red one, blue one, white one, black one etc, so I like to change the wheels on mine so it stands out from everyone elses.

Whether the M3 has 18" or 20" on the track would not make much difference in my case as I am not a professional driver, I could drive on both sets of wheels and do the same times, if worst came to worst put a race exhaust system on the M3 with 20" wheels and it will make up the difference.

I like 20", I say go for it, a don't break the speed limit.

__________________

X5 F15 M50d White, 20"rims, Yellow Calipers, Ceramic Pads. Better off saying what it does not have and that's a B&O Audio System, otherwise all option boxes are ticked.

I think I'm going to have to respectfully disagree with your first point. A 1 inch larger diameter rim that is overall 1lb heavier with 3lbs less wrapped around the outside, rear wheel example, should rotate much easier. The rim will play a much lesser roll than what's wrapped around the outer edge of it because that extra 3lbs is at the farthest point. The 1lb is only an extra .5inch farther from center. The 3lbs of tire is much farther from center and is 3 times heavier than that 1lb of rim. Plus the overall weight of the rim & tire is 2lbs less than the 19.

If you look at the wheel & tire from the center out this is what you get. Take the 20" setup for example. The overall diamter is 26.73 or a radius of 13.365 inches. Zero to 9.5 inches from center they are the same(19 inch diameter). Then the next .5 inch is 1lb heavier. Then the next 3.365 inches is 3lbs lighter than the 19" setup.

I'm not so sure about your second point except for the fact that each car is engineered from the factory to each specific sidewall. Otherwise your argument wouldn't hold any water for the Porsche 911 GT3 or the new Corvette ZR-1. Both of which use tire diameter or sidewall depths which violate what you say. Thinner sidewalls allow the suspension engineers to more accurately dial in a suspension because there is less tire variable to compensate for.

Fair enough. I will respectfully disagree back, but this time with a wealth of evidence to make my case.

Have a look at the table below. I have made a nice spreadsheet (and even attached it for the real nerds) to calculate wheel moment of inertia (among other things). As you can see in your particular cases colums D vs. E and F vs. G the larger wheel (despite not being quite apples to apples in terms of widths) uniformly have MUCH larger moments of inertia. The moments are larger because of the effect of the wheel, not the tire. The larger wheels have a much higher concentration of mass at a large distance away from the center of rotation. I can't quite follow/decipher your "reasoning" above but it is not correct and does not arrive at realistic numbers, trends nor conclusions. Your insight here is simply flawed.

I should add some key assumptions in my calculations (again for all the real nerds that may scrutinize the details):
1. I assumed the cross section of the wheel is a squared "U" on its side, i.e. with no spokes but solid. This is a fair assumption as you can make that section of a uniform thickness that approximates tapered spokes as are present in most real wheels.
2. I also assumed that a tire is a squared U (rotated 90 degrees clockwise to mate with the wheel).
3. I used the "thin shell" volume approximation for the barrel parts of the wheel and tire for volume calculations.
4. The thicknesses of the various sections are reasonable educated guesses, but as you can see they give very reasonable values of component weights, in many places properly predicting which wheel/tire is heavier/lighter compared to your data. As well you can tweak these values around quite a bit and the overall conclusion about I total will not change.
5. Don't worry about the weight prediction being spot on - it really does not matter much. The moment of inertia is the key parameter here and it depends much more on simply predicting where the wheel is in space and where the tire is in space and having those densities close.
6. I used a reasonable value of .02 lb/in^3 density for a tire (note almost 5 times less than aluminum as per my original point!). This value was determined by computing the volume of my simplified U shaped tire and dividing a known tire weight by this.

As far as my second point in my previous post, I'll disagree again. Your counter example doesn't prove what is or is not good for the M3. See comments from footie above. It isn't that you can't make a car work with less sidewall than the very particular M3 18" tire. It just takes engineering the entire suspension and suspension mounting elastomers and other minutia AROUND the particular choice of tire. And, as I said, for the M3 the 18 is ideal.

Once again (for the E9X M3)....

18s for performance
20s for looks
19s for a decent compromise

And of course the driver will be an even bigger factor than these differences.

Fair enough. I will respectfully disagree back, but this time with a wealth of evidence to make my case.

Have a look at the table below. I have made a nice spreadsheet (and even attached it for the real nerds) to calculate wheel moment of inertia (among other things). As you can see in your particular cases colums D vs. E and F vs. G the larger wheel (despite not being quite apples to apples in terms of widths) uniformly have MUCH larger moments of inertia. The moments are larger because of the effect of the wheel, not the tire. The larger wheels have a much higher concentration of mass at a large distance away from the center of rotation. I can't quite follow/decipher your "reasoning" above but it is not correct and does not arrive at realistic numbers, trends nor conclusions. Your insight here is simply flawed.

Once again (for the E9X M3)....

18s for performance
20s for looks
19s for a decent compromise

And of course the driver will be an even bigger factor than these differences.

What if you are going to 20's and the wheels are actually 3-4 lbs. per/wheel lighter than the stock 19s. How does that factor into your inertia equation?

Fair enough. I will respectfully disagree back, but this time with a wealth of evidence to make my case.

Have a look at the table below. I have made a nice spreadsheet (and even attached it for the real nerds) to calculate wheel moment of inertia (among other things). As you can see in your particular cases colums D vs. E and F vs. G the larger wheel (despite not being quite apples to apples in terms of widths) uniformly have MUCH larger moments of inertia. The moments are larger because of the effect of the wheel, not the tire. The larger wheels have a much higher concentration of mass at a large distance away from the center of rotation. I can't quite follow/decipher your "reasoning" above but it is not correct and does not arrive at realistic numbers, trends nor conclusions. Your insight here is simply flawed.

I should add some key assumptions in my calculations (again for all the real nerds that may scrutinize the details):
1. I assumed the cross section of the wheel is a squared "U" on its side, i.e. with no spokes but solid. This is a fair assumption as you can make that section of a uniform thickness that approximates tapered spokes as are present in most real wheels.
2. I also assumed that a tire is a squared U (rotated 90 degrees clockwise to mate with the wheel).
3. I used the "thin shell" volume approximation for the barrel parts of the wheel and tire for volume calculations.
4. The thicknesses of the various sections are reasonable educated guesses, but as you can see they give very reasonable values of component weights, in many places properly predicting which wheel/tire is heavier/lighter compared to your data. As well you can tweak these values around quite a bit and the overall conclusion about I total will not change.
5. Don't worry about the weight prediction being spot on - it really does not matter much. The moment of inertia is the key parameter here and it depends much more on simply predicting where the wheel is in space and where the tire is in space and having those densities close.
6. I used a reasonable value of .02 lb/in^3 density for a tire (note almost 5 times less than aluminum as per my original point!). This value was determined by computing the volume of my simplified U shaped tire and dividing a known tire weight by this.

As far as my second point in my previous post, I'll disagree again. Your counter example doesn't prove what is or is not good for the M3. See comments from footie above. It isn't that you can't make a car work with less sidewall than the very particular M3 18" tire. It just takes engineering the entire suspension and suspension mounting elastomers and other minutia AROUND the particular choice of tire. And, as I said, for the M3 the 18 is ideal.

Once again (for the E9X M3)....

18s for performance
20s for looks
19s for a decent compromise

And of course the driver will be an even bigger factor than these differences.

There are couple of points I would like to make, first about your chart:-

I see a problem in your chart and that is when you go up in rim size to 20" the series size of the tyre drops by 5. 19" wheels with 265/30/19 rears change to 285/25/20. You seem to have used the wrong wheel sizes, 265/30/19 are the right size on a M3 not 265/35/19 as in your chart and the fronts are 245/35/19. In most cases the overall wheel diameter of the tyre does not change and in fact if you went to 275/25/20 the wheel diameter would be smaller than the 265/30/19 but the 20" wheel would be wider. Taking into fact that you could also attain a weight reduction with a 20" combination your chart needs to be recalculated. Your chart simply widens the wheel size & width but not the series size and your start point is wrong. The other fact is that a Michellin tyre is actually 5mm wider than its actual size and your will find that with quite a few tyre companies tyres. I could not see anyone running a 295/30/19, you would run a 295/25/19 to keep the rolling diameter the same. The difference between a 265/30/19 (79.5 side wall) and a 295/25/20 (73.75 side wall) is only around 5.5mm and with tyres not being exact widths as stated that maybe only 2-3mm side wall difference.

Further, is straight line drag from 0-100mph the 20" M3 because of more rubber on the ground would be quicker with LC off or on because it could lay more of its power down because there is more power in the M3 than the 18" tyres can handle.

As far as I can see by your chart your are telling me there is a difference between 19" & 20" wheels when in fact you could be presented with a 20" wheel that maybe only 5mm wider, have smaller rolling diameter and be lighter.

Second:-

I do take onboard your statement that the M3 was (1) setup for 18" & (2) test results were with 18" but thats now the car was tested. I do not see any evidence that was how the car is sold to us. I do not see BMW tunning the car to 18" and then selling an option of 19", nearly all the M3 coming to Australia have 19". I believe the M3 is setup to run both on 18" & 19", the tyre choice of 18" is more to do with the condition of roads in some countries and the 18" being softer tyre to ride on & helps with rim damage. Oh yes don't forget snow chains? hey can you put snow chains on a M3?

I do agree that there is a fair bit of difference between a 18" & 20" wheels, but I believe the HP of the car plays a great part. Here we have a V8 with a e-diff running the same tyres as the e46 M3, the other point is that the DTC 7 speed is quicker than the 6MT.

Now if the M3 has power to spare then 19" would be a benefit. The DTC M3 is faster than a 6MT M3, 3rd gear is less of a problem with the 7 speed DTC, I bet it runs better on 19" than 18".

Your point rests with the fact that you state the M3 was designed around 18" wheels and tested around 18" wheels thats fine, but I say its not sold to us in that format, its been compromised, its been sold to us to run both 18" & 19", its also de-turned for different countries, due to both fuel quality & sub-tropical climates. I don'e see anywhere in the manual that the car performs better on 18" then 19' and I do not see any warnings that the handling will be compromised if you put 19" on. I also don't see anywhere where it says for best track times use 18". If anything the suspension has been beefed up to handle 19" not vice versa.

My principal is give me more power then give me more rubber, I don't drive on a track I drive on a road and I probably apply the brakes more than the accelerator anyway.

Anyway bring it on, I'm waiting?

PS. On M3 test track days I asked why they were running on 18", I was told because the car was more forgiving for the average driver, the tyres were promotional from the tyre company and if they used more than they had they were cheaper than the 19" to buy, they also had less damaged rims.

__________________

X5 F15 M50d White, 20"rims, Yellow Calipers, Ceramic Pads. Better off saying what it does not have and that's a B&O Audio System, otherwise all option boxes are ticked.

There are couple of points I would like to make, first about your chart:-

I see a problem in your chart and that is when you go up in rim size to 20" the series size of the tyre drops by 5. 19" wheels with 265/30/19 rears change to 285/25/20. You seem to have used the wrong wheel sizes, 265/30/19 are the right size on a M3 not 265/35/19 as in your chart and the fronts are 245/35/19. In most cases the overall wheel diameter of the tyre does not change and in fact if you went to 275/25/20 the wheel diameter would be smaller than the 265/30/19 but the 20" wheel would be wider. Taking into fact that you could also attain a weight reduction with a 20" combination your chart needs to be recalculated. Your chart simply widens the wheel size & width but not the series size and your start point is wrong. The other fact is that a Michellin tyre is actually 5mm wider than its actual size and your will find that with quite a few tyre companies tyres. I could not see anyone running a 295/30/19, you would run a 295/25/19 to keep the rolling diameter the same. The difference between a 265/30/19 (79.5 side wall) and a 295/25/20 (73.75 side wall) is only around 5.5mm and with tyres not being exact widths as stated that maybe only 2-3mm side wall difference.

Further, is straight line drag from 0-100mph the 20" M3 because of more rubber on the ground would be quicker with LC off or on because it could lay more of its power down because there is more power in the M3 than the 18" tyres can handle.

As far as I can see by your chart your are telling me there is a difference between 19" & 20" wheels when in fact you could be presented with a 20" wheel that maybe only 5mm wider, have smaller rolling diameter and be lighter.

Second:-

I do take onboard your statement that the M3 was (1) setup for 18" & (2) test results were with 18" but thats now the car was tested. I do not see any evidence that was how the car is sold to us. I do not see BMW tunning the car to 18" and then selling an option of 19", nearly all the M3 coming to Australia have 19". I believe the M3 is setup to run both on 18" & 19", the tyre choice of 18" is more to do with the condition of roads in some countries and the 18" being softer tyre to ride on & helps with rim damage. Oh yes don't forget snow chains? hey can you put snow chains on a M3?

I do agree that there is a fair bit of difference between a 18" & 20" wheels, but I believe the HP of the car plays a great part. Here we have a V8 with a e-diff running the same tyres as the e46 M3, the other point is that the DTC 7 speed is quicker than the 6MT.

Now if the M3 has power to spare then 19" would be a benefit. The DTC M3 is faster than a 6MT M3, 3rd gear is less of a problem with the 7 speed DTC, I bet it runs better on 19" than 18".

Your point rests with the fact that you state the M3 was designed around 18" wheels and tested around 18" wheels thats fine, but I say its not sold to us in that format, its been compromised, its been sold to us to run both 18" & 19", its also de-turned for different countries, due to both fuel quality & sub-tropical climates. I don'e see anywhere in the manual that the car performs better on 18" then 19' and I do not see any warnings that the handling will be compromised if you put 19" on. I also don't see anywhere where it says for best track times use 18". If anything the suspension has been beefed up to handle 19" not vice versa.

My principal is give me more power then give me more rubber, I don't drive on a track I drive on a road and I probably apply the brakes more than the accelerator anyway.

Anyway bring it on, I'm waiting?

PS. On M3 test track days I asked why they were running on 18", I was told because the car was more forgiving for the average driver, the tyres were promotional from the tyre company and if they used more than they had they were cheaper than the 19" to buy, they also had less damaged rims.

It is pretty simple. Column A is the OEM 19" rear. Column B is a 20 inch rear option that maintains the same overall OD, which is typically what is done when replacing wheels and tires. The other columns are just the specific sizes mentioned by the OP. I did realize that those were not always an "apples to apples" comparison not do they maintain a fixed overall OD.

Go ahead and download the zip file with the spreadsheet inside - play around with some numbers. My point of this exercise was not to model EXACTLY any specific wheel and tire. If I wanted precise I's I would simple make a 3D CAD model and get the moments of inetria that way. The point of the spreadsheet and going through the calculation was simply to demonstrate that larger wheel diameters will almost universally hurt you when it comes to a much larger I. For rotating mass I is more important than mass and is typically overlooked. The other point the calculation makes is that even a lighter wheel and tire combination can have a larger I than a lighter combination.

These conclusions are quite solid.

As far as dragging I would not bet on the 20s over the 18s. Sidewall flex is as critical in draging as in a road course. Of course tire width is well understood, the wider the tire typically the better. The sidewall size and tire width and completely independent effects.

As far as the way some countries option their cars: That will happen. The difference between an 18 and a 19 is going to be smaller than the difference between and medicore and good driver. It is not an enormous effect, but it is there. I'm sure some track nuts in your neck of the woods specifically requested the 18s.

20" wheels and tires on the Turner car are for show, they want to sell you big wheels with enormous staggers, but I seriously doubt they would sell you this for any sort of performance advantage on the track or even on the street.